1. Field of the Invention
The present invention is directed to a spectral scanning radiometer which can detect the presence of and the intensity variations of radiant energy in specific wave bands, such as the visible, near infrared, and ultraviolet wave bands. More particularly, the present invention is directed to an optical pathway of radiant energy in a computerized spectral scanning radiometer.
2. Discussion of the Background
In the environment, solar light plays a key role in seasonal control of growth and development of plants which are harvested for food and medicinal products. In fact, all biological systems in one way or another are dependent upon light for survival. In recent years, environmental concern has focused on the amount of ultraviolet radiation (UV) passing through the Earth's atmosphere due to shifts in the thickness of the protective ozone layer. UV measurements are routinely taken at various places to determine the "sunburn index" at that location to offer warnings to the general public. In industry, the measurement of UV radiation is used for wide ranging applications from the production of chemical fluorocarbons, to aiding the growth of plants in natural and artificial environments, to the production of natural products, and from bioreactors to the testing of light effects on materials such as paints, coatings, building materials, etc.
A thorough understanding of the effects of irradiance, and particularly UV irradiance, and the role it plays in environmental, agricultural and industrial processes is vitally important. In plants, for example, specific photo processes are initiated by the absorption of radiant energy in specific wavebands of UV, visible and near visible IR spectrum. In order to understand the effects that minute differences in the wavebands of radiant energy may have on these processes, it is important to accurately quantitate the irradiance spectrum in the field and in the laboratory.
Measurements of the energy of the sun (solar radiation) reaching the surface of the earth have been undertaken for more than 100 years using a number of techniques and instrument designs. One observation method utilizes an instrument design which "sees" not only the direct beam of light from the sun, but also all solar radiation scattered toward the earth by the earth's atmosphere as well. The usual method of measuring this "sun and sky" radiation was by an instrument whose collector was a metallic flat plate mounted in a horizontal position, and coated with a solar energy absorbing compound. As the plate heated from the absorbed solar radiation, the temperature increase could be measured and quantified. With a calibrated instrument of this type, quite accurate measurements of solar energy could be obtained.
The energy absorbed by a horizontal flat plate collector decreases as the angle of the energy source moves away from the zenith (directly overhead) even though the distance from the source to the collector remains constant. This change in absorbed energy is proportional to the cosine of the angle of the source to the detector's zenith (zenith angle). This effect is referred to as the cosine response of the instrument. A very great number of readings have been made and published using a wide variety of flat plate type collectors.
Developments in instrument design utilizing different detectors and using filters to narrow the regions of the solar spectrum detected by the instrument require that those components be mounted inside the instrument case. Because of the long history of the use of flat plate collectors, there is a need for instruments of other designs to mimic the flat plate collector cosine response to the angle of incident light.
Since the early 1970's the Smithsonian Institution has been developing spectral radiometers designed to monitor solar radiation in narrow spectral bands. These instruments use detectors and filters mounted well inside the instrument. This configuration required the design of a light entrance port window which could mimic the cosine response of a flat plate detector. The port design has to address two problems, because of which problems a transparent flat plate is an unsuitable window. The first problem is that detectors such as photomultiplier tubes (PMTs) have a small active surface which is sensitive and light arising from varying points will have different "views" of the detector. This positional dependence needs to be removed. Second, because of the position of the detector deep within the instrument, rays at high zenith angles will not reach the detector. Characteristic of the first problem is quadrant dependent response, and characteristic of the second problem is failure of cosine response at high zenith angles of incident light. These two technical problems have been addressed in the past by (1) using a diffusing material which will randomize light propagation removing the position (or quadrant) dependency, and (2) introducing an elevated contour on the diffuser surface to more efficiently capture light at high zenith angles.
The optical properties of this window material and the diffuser shape are critical for proper duplication of a correct instrument cosine response, particularly in the UVB range. The material must be able to diffuse the light falling upon it as well as transmit the wavelengths of light desired to be measured. The material needs to diffuse the light falling on it, such that light incident from any angle would be diffused through the window in a direction toward the instrument filters and detector. The material must also be able to transmit the wavelengths of light which are desired to be measured.
The shape of the diffuser is also critical for proper cosine response. This is particularly important with respect to obtaining a proper cosine response for UVB radiation. UVB light will come in from all angles, as it is widely scattered. Optical properties of filters in an optical path also require collimation of the light passing through the window in order to understand the instrument spectral response.